The invention relates to non-contact techniques for determining the position in space of points on an object part, including sub-micron precision height measurements of a selected part surface. Example applications are surface profiling, the determination of the height of surface features, which may include dissimilar materials, and the measurement of surface features with respect to a fixed coordinate system or datum so as to determine part location and orientation.
Various optical sensing technologies are available for measuring surface profiles. Height scanning interferometry, as referred to herein, employs broadband light sources to determine 3-D surface height profiles without the fringe-order ambiguity normally associated with laser-based interferometry. In the visible, this height scanning is often referred to in the art as scanning white light interferometry (SWLI), coherence radar, correlation microscopy and vertical scanning interferometry.
The invention features methods and systems that extend the basic technique of height scanning interferometry by compensating for the phase change on reflection (PCOR) caused by the interferometer and the object part being measured, including the wavelength dependence (dispersion) of PCOR. Proper compensation of such effects improves the accuracy of scanning height measurements. For example, object parts having regions with different optical properties can be more accurately characterized.
Also, the invention extends height-scanning interferometry to those metrology problems for which the absolute position and orientation of the surface is as important as the surface texture and profile. Thus, in addition to providing the form and texture of the surface, the overall position (piston), tip and tilt of the object part can be determined with respect to a fixed point or plane of reference when constructing the surface profile.
In general, in one aspect, the invention features a surface profiling method for determining the relative positions of multiple points on an object surface including multiple surface materials. The method includes: collecting interferometric data related to the relative positions; and calculating the relative positions based on the collected interferometric data and at least one value indicative of dispersion in the phase change on reflection (PCOR) for each of the surface materials.
In general, in another aspect, the invention features a surface profiling method for determining the relative positions of multiple points on an object surface including multiple surface materials. The method includes: collecting interferometric data related to the relative positions; and calculating the relative positions based on the collected interferometric data and at least one value indicative of the phase change on reflection (PCOR) xcex3part for each of the surface materials.
In general, in another aspect, the invention features a surface profiling method for determining the absolute position with respect to a common datum surface of each of multiple points on an object surface. The method includes: collecting interferometric data related to the absolute positions; and calculating the absolute positions based on the collected interferometric data and at least one value indicative of dispersion in the phase change on reflection (PCOR) of the object surface and dispersion in the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data.
In general, in another aspect, the invention features a surface profiling method for determining the absolute position with respect to a common datum surface of each of multiple points on an object surface. The method includes: collecting interferometric data related to the absolute positions; and calculating the absolute positions based on the collected interferometric data and at least one value indicative of the phase change on reflection (PCOR) of the object surface xcex3part and the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data xcex3sys.
In general, in another aspect, the invention features a surface profiling method including: collecting interferometric data related to a surface profile of a measurement object; and calculating the surface profile based on the collected interferometric data and at least one value indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object.
Embodiments of this aspect of the invention can include any of the following features.
The calculation of the surface profile can be based on the collected interferometric data and multiple values indicative of dispersion in the phase change on reflection (PCOR) of different regions of the profiled surface of the measurement object.
The calculation of the surface profile can be based on the collected interferometric data and at least one value indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object and dispersion in the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data.
The collection of interferometric data can include collecting scanning interferometric data related to the surface profile of the measurement object.
The collected interferometric data can include at least one phase value xc3x8(k) at a wavevector k for each of multiple points on the profiled surface, and wherein the calculation of the surface profile is based on the relationship
xc3x8(k)=2nk(hxe2x88x92xcex6)+(xcex3part+xcex3sys)+(xcfx84part+xcfx84sys)(kxe2x88x92k0)
where h is the surface height, xcex6 is a reference offset position, n is a refractive index, xcex3part is the phase change on reflection (PCOR) of the profiled surface at the wavevector k, xcex3sys is the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data at the wavevector k, xcfx84part is the linear dispersion of the phase change on reflection (PCOR) of the profiled surface with respect to a reference wavevector k0, and xcfx84sys is the linear dispersion of the phase change on reflection (PCOR) of the interferometer system with respect to wavevector k0.
The collected interferometric data can include multiple phase values xc3x8(k) corresponding to multiple wavevector values k for each of multiple points on the profiled surface, and wherein the calculation of the surface profile is based on the relationship             ∂              φ        ⁡                  (          k          )                            ∂      k        =            2      ⁢              n        ⁡                  (                      h            -            ζ                    )                      +          (                        τ          part                +                  τ          sys                    )      
where h is the surface height, xcex6 is a reference offset position, n is a refractive index, xcfx84part is the linear dispersion of the phase change on reflection (PCOR) of the profiled surface with respect to a reference wavevector k0, and xcfx84sys is the linear dispersion of the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data with respect to the reference wavevector k0.
The interferometric data can be scanning interferometric data including multiple phase values xc3x8(xcex6) corresponding to multiple positions xcex6 of a scanning reference mirror for each of multiple points on the profiled surface, and the calculation of the surface profile includes transforming the multiple phase values for each point into a wavevector domain, calculating a derivative of the transformed phase values for each point with respect to wavevector, and calculating the surface profile from the derivative for each point and the at least one value indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object.
In such embodiments, the at least one value can be indicative of the dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object and the dispersion in the phase change on reflection (PCOR) of a scanning interferometry system used to collect the interferometric data, and the calculated surface profile provides an absolute position with respect to a common datum surface for each of the multiple points on the profiled surface. he interferometric data includes coherence profiling data and phase profiling data, and wherein the calculation of the surface profile includes using the coherence profiling data and the at least one value indicative of the dispersion in the phase change on reflection (PCOR) to determine an initial estimate of the surface profile, and using the initial estimate to resolve fringe ambiguity in the phase profiling data.
The at least one value can be xcfx84part+xcfx84sys, where xcfx84part is the linear dispersion of the phase change on reflection (PCOR) of the profiled surface with respect to a reference wavevector k0, and xcfx84sys is the linear dispersion of the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data with respect to the reference wavevector k0.
The at least one value can include xcfx84part and xcfx84sys, where xcfx84part is the linear dispersion of the phase change on reflection (PCOR) of the profiled surface with respect to a reference wavevector k0, and xcfx84sys is the linear dispersion of the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data with respect to the reference wavevector k0.
In such embodiments, the at least one value can further include xcex3part and xcex3sys, where xcex3part is the phase change on reflection (PCOR) of the profiled surface at a particular wavevector k, and xcex3sys is the phase change on reflection (PCOR) of the interferometry system at the wavevector k.
The at least one value can include a first value indicative of the dispersion in the phase change on reflection (PCOR) of the profiled surface and a second value indicative of a phase change on reflection (PCOR) of the profiled surface for a particular wavevector k.
The calculated surface profile can provide an absolute position with respect to a common datum surface of each of multiple points on the profiled surface.
The collected interferometric data can include coherence profiling intensity data as a function of a reference mirror scan position xcex6, wherein the calculation of the surface profile is based on the relationship
h=xcex6maxxe2x88x92(xcfx84sysxe2x88x92xcfx84part)/2n
where h is the surface height, xcex6max is the reference scan position where the intensity data is maximized, n is a refractive index, xcfx84part is the linear dispersion of the phase change on reflection (PCOR) of the profiled surface with respect to a reference wavevector k0, and xcfx84sys is the linear dispersion of the phase change on reflection (PCOR) of an interferometry system used to collect the interferometric data with respect to the reference wavevector k0.
In general, in another aspect, the invention features a surface profiling system including: an interferometry system which during operation provides interferometric data related to a surface profile of a measurement object; and an electronic processor coupled the interferometry system, wherein during operation the electronic processor calculates the surface profile based on the interferometric data and at least one parameter indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object.
Embodiments of this aspect of the invention can include any of the features described above for the surface profiling method and also any of the following features.
The at least one parameter can be indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object and dispersion in the phase change on reflection (PCOR) of the interferometry system, and wherein the surface profile calculated by the electronic processor includes an absolute position with respect to a common datum surface of each of multiple points on the profiled surface.
The electronic processor can calculate the surface profile based on the interferometric data and parameters indicative of dispersion in the phase change on reflection (PCOR) of different surface materials of the profiled surface.
The interferometry system includes: a broadband source; a scanning interferometer which during operation directs a first wavefront derived from the source along a reference path and a second wavefront derived from the source along a measurement path contacting the measurement object, and, after the second wavefront contacts the measurement object, combines the wavefronts to produce an optical interference pattern; a detector producing the interference data in response to the optical interference pattern; and a scanning controller coupled to the interferometer and the detector, which during operation causes the scanning interferometer to vary the optical path difference between the reference and measurement paths.
In general, in another aspect, the invention features a method for calibrating an interferometry system to determine at least one of the phase change on reflection (PCOR) of the interferometry system xcex3sys and linear dispersion of the phase change of reflection (PCOR) of the interferometry system xcfx84sys with respect to a reference wavevector k0. The calibration method includes: collecting interferometric data relating to a test surface using the interferometry system; providing independent information for the profile h of the test surface, the phase change on reflection (PCOR) of the test surface xcex3part, and the linear dispersion of the phase change on reflection (PCOR) of the test surface xcfx84part with respect to a reference wavevector k0; and calculating the at least one of the phase change on reflection (PCOR) of the interferometry system xcex3sys and the linear dispersion of the phase change of reflection (PCOR) of the interferometry system xcfx84sys based on the interferometric data and the independent information.
Embodiments of this aspect of the invention can include any of the following features.
The collected interferometric data includes at least one phase value xc3x8(k) at a wavevector k for each of multiple points on the test surface, and wherein the calculation of the surface profile is based on the relationship
xc3x8(k)=2nk(hxe2x88x92xcex6)+(xcex3part+xcex3sys)+(xcfx84part+xcfx84sys)(kxe2x88x92k0)
where xcex6 is a reference offset position and n is a refractive index.
The at least one of xcex3sys and xcfx84sys can include xcex3sys.
The at least one of xcex3and xcfx84sys can include xcfx84sys .
The at least one of xcex3sys and xcfx84sys can include xcex3sys and xcfx84sys, the interferometric data can provide an experimentally observed phase gap Gex, and the calculation of xcfx84sys is based on the relationship
xcfx84sys=[(xcex3part+xcex3sysxe2x88x92Gex)/k0]xe2x88x92xcfx84part.
In general, in another aspect, the invention features a method for measuring at least one of the phase change on reflection (PCOR) of test material xcex3part and linear dispersion of the phase change of reflection (PCOR) of the test material xcfx84part with respect to a reference wavevector k0. The method includes: collecting interferometric data relating to a test surface using an interferometry system; providing independent information for the phase change on reflection (PCOR) of the interferometry system xcex3sys and the linear dispersion of the phase change on reflection (PCOR) of the interferometry system xcfx84sys with respect to a reference wavevector k0; and calculating the at least one of the phase change on reflection (PCOR) of the test material xcex3part and the linear dispersion of the phase change of reflection (PCOR) of the test material xcfx84part based on the interferometric data and the independent information. In some embodiments, the at least one of xcex3part and xcfx84part includes xcfx84part, the interferometric data provides an experimentally observed phase gap Gex, and the calculation of xcfx84part is based on the relationship
xcfx84part=[(xcex3part+xcex3sysxe2x88x92Gex)/k0]xe2x88x92xcfx84sys.
Other aspects, advantages, and features of the invention follow.